Several prior art communications systems, such as the paging system (FIG. 1A) described in U.S. Pat. No. 5,274,579 (granted to Nelson et al, and which is incorporated by reference herein in its entirety), include a circuit (commonly called xe2x80x9ctone detectorxe2x80x9d) 10 to detect the presence of a signal (also called xe2x80x9ctone signalxe2x80x9d) that has a known frequency. As described therein, an analog circuit 11 (FIG. 1B) can be used to convolute (multiply and accumulate) input signal with analog sine and cosine reference signals, to obtain two components Vos and Voc (FIG. 1C). The two components are then squared (in devices 12A and 12B of FIG. 1B), and the squares summed (in summer 13), followed by a square-root operation on the sum (in device 14) to obtain the magnitude Vo of the tone signal, and the magnitude Vo is compared with a threshold Vr (FIG. 1B) in a threshold detector 15 to determine if the tone signal is present. Note that a tone signal is detected in the example illustrated in FIG. 1C even though Vos and Voc are less than Vr, because Vo is greater than Vr. In FIG. 1C, Vm denotes the maximum magnitude possible for the input signal, and Vn denotes noise.
Another patent, U.S. Pat. No. 4,302,817 (granted to Labdez, and which is incorporated by reference herein in its entirety), describes a tone detector having an optional filter (which is preferably a low pass or band pass filter), followed by a limiter (which hard limits the signal thus converting the signal into essentially a rectangular waveform). The output of the limiter is supplied to a control logic (the circuit illustrated in FIG. 1D) which has been modeled by Nelson et al. (incorporated by reference in the above paragraph), and which uses square wave reference signals SQS (Wt) and SQS (Wt) having one of two values +1 and xe2x88x921 for digital processing. Also according to Nelson et al., conventional analog multiplications were replaced by simple Boolean multiplications, conventional analog integrators were replaced by summers, the squaring functions were replaced by absolute value functions, the outputs were digitally summed, and the square root was replaced by a simple divide by two function. In actual practice, the divide by two function was generally omitted, and the output of the summer was directly compared to a threshold, to generate an output signal having a logic value.
Nelson et al. indicate that the Labdez tone detector (which includes the circuit of FIG. 1D) had increased sidelobe responses (due to the rectangular observation window), and also had increased harmonic responses, particularly third harmonic responses (due to the rectangular reference signals at the tone frequency). Yet another patent, U.S. Pat. No. 4,513,385 (granted to Muri and which is incorporated by reference herein in its entirety), describes use of a rectangular observation window that omits sampling the input signal during a predetermined time interval. Nelson et al. state that Muri""s observation window is suitable for protocols having twelve evenly spaced tones between 350 Hz and 3100 Hz, but unsuitable for decoding sixty tones spaced at 2.77% frequency intervals between 280 Hz and 3100 Hz.
Still another patent, U.S. Pat. No. 5,850,438 (granted to Braams et al, and which is incorporated by reference herein in its entirety), includes a number of correlators 16A-16D (FIG. 1E) that measure the input signal during mutually displaced measuring periods. The additional correlators ensure that at least one correlator always covers the complete period during which the tone is present. A selector 17 selects one of correlators 16A-16D (depending on whichever has the largest value) to be coupled to a comparator 18 that compares the output from the correlator with a threshold to detect the presence of the tone signal. One implementation (FIG. 1F) of a single correlator 16I includes four correlating elements 19A-19D, and each correlating element includes an exclusive NOR gate that receives a reference signal and the input signal, and supplies an output signal of value xe2x80x9c1xe2x80x9d when both signals have equal values and otherwise supplies a xe2x80x9c0xe2x80x9d, and this output signal controls the direction of an up/down counter. Each of the four up/down counters are coupled to an adder 20 that in turn is coupled to a comparator, for comparison of the adder""s output to a threshold value (FIG. 1E).
Note that Braams"" correlating elements 19A-19D (FIG. 1F) receive reference signals that have a phase difference of xcfx80/4. Braams et al. state that xe2x80x9c[a] correlation signal substantially different from zero will be present if the input signal comprises a signaling tone with a frequency corresponding to the frequency of the reference signal. The tolerated frequency difference depends on the measuring time used. This measuring time used is defined by the time between the two subsequent reset instants of the integrators . . . A finite measuring time Tm results in a rectangular window function applied to the output signal of the multipliers . . . By choosing a suitable value for the measuring time every desired frequency resolution can be obtained. Because the measuring time Tm can easily be changed, the frequency resolution can easily be changed too. By using four reference signals having a phase incrementing by xcfx80/4 always a major correlation signal is generated, irrespective of the phase of the reference tone. It can be shown that the amplitude variation of the combined correlation signal as a function of the phase of the signaling tone, is not more than 10%.xe2x80x9d
See also the article entitled xe2x80x9cA Digital Receiver for Tone Detection Applicationsxe2x80x9d by Theo A. C. M. Classen and J. B. H. Park, in IEEE Transactions on Communications, Vol. Com-24, No. 12, December 1976, pp. 1291-1300.
In accordance with the invention, a tone detector includes at least one circuit (hereinafter xe2x80x9csingle phase reference matcherxe2x80x9d) that not only performs a convolution of an input signal with a reference signal, but also compares the result of convolution with a threshold to determine if there is a match, and if so drives a signal active indicating that tone is present in the input signal. Therefore, the just-described circuit does not combine the results of convolution of the input signal with reference signals of multiple phases prior to comparison with the threshold. Instead, the single phase reference matcher performs in a single operation the just-described acts of convolution and comparison, to detect a match between a single reference signal and the input signal.
If there is no match, another circuit (hereinafter xe2x80x9cphase shifterxe2x80x9d) delays the reference signal by a fraction (e.g. xe2x85x9) of the measuring period, thereby to introduce a phase shift (e.g. xcfx80/8) between the input signal and the reference signal. The single phase reference matcher performs the just-described operation with one or more of such delayed reference signals as often as necessary (e.g. eight times) to cover the entire measurement period, thereby to ensure that tone (if present in the input signal) is detected irrespective of phase, during one of the operations. The smaller the phase shift, the greater the number of times the operation is repeated, and vice versa.
A tone detector of the type described herein can include more than one single phase reference matcher, each of which performs the above-described operation in parallel, thereby to reduce the total time required. In such a case, each of the single phase reference matcher (also called simply xe2x80x9cphase matchersxe2x80x9d) has its output port coupled to an OR gate, and the OR gate indicates presence of tone irrespective of which of the phase matchers found the tone. For example, if the tone detector includes two phase matchers, then all the operations (to cover a measuring period) can be performed in half of the time required by having only one phase matcher. In one extreme example, the just-described operations are all performed simultaneously (by multiple phase matchers equal in number to the number of operations), so that tone is detected at the end of the measuring period.
Note that comparison between convolution with a single reference signal and a threshold can result in false detection of tone due to noise. Therefore, in one embodiment, the measuring period is deliberately selected to be long enough to include a number xe2x80x9ccxe2x80x9d of cycles of the input signal (at the tone frequency), to reduce noise that would otherwise be present when using a measuring period that covers a single cycle. The number xe2x80x9ccxe2x80x9d is chosen to suppress the contributions of tone signals at adjacent frequencies, for example by selecting c to be (Sf/xcex94f) or some multiple thereof, wherein xcex94f is the difference between the tone frequency of interest and the nearest frequency at which another tone may be present, and Sf is the sampling frequency.